Conventionally, a compact power inductor (chip inductor) has been used for realizing functions such as suppressing noise, rectification, and smoothing signals, for example, in a power supply circuit for semiconductors, a power circuit for a DC-DC convertor, and so on. A large inductance and a low resistance are required for the compact power inductor.
One of the compact power inductors is a layered inductor (multi-layered inductor). As shown in FIG. 36, which is a perspective view of the layered inductor 110, the layered inductor 110 typically comprises a magnetic body 111 and a coil 112 buried in the magnetic body 111.
The coil 112 comprises conductive layers (conductive thin plates) 112A, each of which is formed so as to have a predetermined shape, and conductive connecting portions 112B in via holes (VIA), each of which electrically via-connects (wiring-connects between layers) between the conductive layers adjacent to each other in a direction of layering (in a vertical direction), and is formed to have a helical shape. This type of layered inductor 110 is manufactured, for example, by printing and layering method, tape layering method, and the like.
In order to lower a resistance in such a layered inductor 110, it is necessary to increase a cross sectional area of the coil 112. However, as shown in FIG. 37, according to the printing and layering method, the tape layering method, or the like, magnetic-layers-before-fired 111a are firstly prepared and a conductive-layer-before-fired 112a is formed on each of the magnetic layers 111a. Subsequently, as shown in FIG. 38, they are layered/laminated to form a layered body, and thereafter, the layered body is fired (burnt/sintered). Accordingly, as shown in FIG. 38, thickening the conductive layer 112a in order to increase the cross sectional area of the coil 112 leads to a large difference between a thickness X1 of a portion where the conductive layers 112a are formed and a thickness X2 of a portion where no conductive layer 112a is formed.
As a result, when the layered body is fired, a gap (a crack) may be generated/occurred between the magnetic layers 111a, 111a adjacent to each other in the direction of layering, and in some cases, a problem that a structural defect may occur arises, e.g., one of the magnetic layers 111a is delaminated (or removed) from another layer 111a adjacent to the one (i.e., a delamination occurs). Further, there may be another problem that the inductor does not have desired electrical characteristics, such as a great decrease in the inductance, due to the structural defect.
In order to cope with the problems, it is proposed that gaps around the conductive layers 112a are formed beforehand (refer to, for example, Patent document 1 and Patent document 2). However, it is necessary to increase a cross sectional area of the gap as the cross sectional area of the conductive layer 112a increases. As a result, it is still difficult to decrease the resistance of the inductor without an excessive decrease of the inductance of the inductor, even by the proposed technique.    Patent Document 1: Japanese Patent No. 2987176    Patent Document 21: Japanese Patent No. 4020131